1. Technical Field of the Invention
The present invention generally relates to the telecommunications field and, in particular, to an apparatus and method that minimizes the computational load in a receiver by adjusting the number of taps used in a pre-filter and equalizer.
2. Description of Related Art
In the telecommunications field, one of the most significant design challenges involves the development of new ways to improve the quality of a received signal. The quality of the received signal is adversely affected by intersymbol interference (ISI) which is often recognized as one of the major obstacles to high speed data transmission over mobile radio channels. ISI is attributable to multipath within time dispersive radio channels and results in the distortion of a transmitted signal that causes bit errors in the received signal at a receiver.
The most common way to compensate for ISI is to use some type of equalizer in the receiver. One of the most desirable or optimal types of equalizers available is a maximum likelihood sequence estimation (MLSE) equalizer also known as a Viterbi decoder. Basically, the MLSE tests all possible data sequences (rather than decoding each received symbol by itself) and chooses the data sequence with the maximum probability as the output. However, the MLSE equalizer can be too complex for practical use in some types of communications systems or receivers.
Therefore, different types of equalizers which are approximations of the MLSE equalizer have been developed to enable the selection of a suitable equalizer when choosing between performance and complexity. Two of these different types of equalizers currently available include a decision feedback equalizer (DFE) and a delayed decision feedback sequence estimator (DFSE).
The basic idea behind the DFE is that once the received signal has been detected and decided upon, the ISI that it induces on future signals can be estimated and subtracted out before detection of subsequent signals. Whereas, the DFSE uses both the MLSE and DFE techniques to compensate for the ISI introduced in the radio channel. In other words, the DFSE uses the MLSE technique to compensate for some of the ISI introduced and the DFE technique is used to compensate for the rest of the ISI.
In addition, the receiver typically utilizes a pre-filter to filter the received signal before it is input to the equalizer. The pre-filter operates to concentrate the energy into channel taps handled by the MLSE (in the DFSE case) or to concentrate the energy into the first channel tap of the DFE (in the DFE case). The traditional DFSE and traditional pre-filter are both described in greater detail below with respect to FIGS. 1-3.
Referring to FIG. 1, there is a block diagram illustrating the basic components of a conventional communications system 100. The communications system 100 includes a transmitter 102 that receives an original message u(t) and transmits the original message on a radio channel 104 to a receiver front end 106. In addition to receiving the transmitted original message u(t), the receiver front end 106 also receives a noise element e(t).
The receiver front end 106 forwards the transmitted original message u(t) and the noise element e(t) to a receiver filter 108. The receiver filter 108 filters the transmitted original message u(t) and the noise element e(t) before an analog-to-digital convertor 110 converts the filtered original message u(t) and noise element e(t) to a received signal y(t). At this point, the received signal y(t) can be represented as either one of the two equations below:
y(t)=h(t)*u(t)+e(t), (t=1, . . . , T)xe2x80x83xe2x80x83(1)
y(t)=xcexa3nu(txe2x88x92n)h(n)+e(t), (t=1, . . . , T)xe2x80x83xe2x80x83(2)
where y(t) is the received signal; h is an unknown radio channel; u(t) is the original message; e(t) is the noise element; n is the total number of channel taps; and T is the number of received samples in a burst 200 (see FIG. 2). For instance, the burst 200 can be a typical Time Division Multiple Access (TDMA) burst including a training sequence 202 that is located between data 204 which are located between tails 206.
The received signal y(t) is input to a channel estimator 112 that operates to estimate the number of channel filter taps ĥ by correlating the received signal y(t) with the known training sequence 202 within the burst 200. The output of the channel estimator 112 includes the information parts of the received signal y(t) which are the estimated number of channel filter taps ĥ and an estimated noise effect {circumflex over ("sgr")}2. The estimated channel filter taps ĥ, the estimated noise effect {circumflex over ("sgr")}2 and the received signal y(t) are input to a pre-filter 114. Referring to FIG. 3A, there is an exemplary graph illustrating the signal strengths of the estimated channel filter taps ĥ before the pre-filter process.
The pre-filter 114, with a fixed number of taps, here called g(t), performs a pre-filter tap calculation based on the estimated channel filter taps ĥ and the estimated noise effect {circumflex over ("sgr")}2 so that the signal energy (e.g., absolute value of the channel filter taps) is concentrated to the first channel taps in the filtered version of the estimated channel taps {tilde over (h)} (e.g., {tilde over (h)}(t)=g(t)*ĥ(t)). Referring to FIG. 3B, there is an exemplary graph illustrating the signal strengths of the filtered channel taps {tilde over (h)} after the pre-filter process.
An equalizer 116 receives the filtered channel taps {tilde over (h)}, the filtered received signal {tilde over (y)}(t) (e.g., {tilde over (y)}(t)=g(t)*ĥ(t)+g(t)*e(t)) and the transformed noise effect {tilde over ("sgr")}2. The equalizer 116 (e.g., DFSE/DFE) has a set number of equalizer taps and operates to output both a symbol and soft information. The symbol is an estimation of the original message u(t). And, the soft information is a reliability measure of the estimated symbol or bits forming the estimated symbol.
To reduce ISI and improve the quality of the received signal, it is desirable to have as many pre-filter taps and equalizer taps as possible within the receiver. Unfortunately, as the number of pre-filter taps and equalizer taps increase, there is a corresponding and problematic increase in the computational load within the receiver. Moreover, the increase in the computational load within the receiver also causes an increase in the overall power consumption which can also be problematic whenever the receiver is incorporated within a mobile phone. Therefore, there is a need for an apparatus and method that effectively minimizes the computational load and power consumption in a receiver while improving the quality of a received signal by reducing ISI.
The present invention is an apparatus and method that effectively minimizes the computational load and reduces the overall power consumption in a receiver by adjusting the number of taps used in a pre-filter and equalizer. More specifically, the apparatus includes a memory for storing a signal, and a channel estimator for estimating a quality parameter and a number of channel filter taps using the stored signal. The apparatus further includes a controller for evaluating the estimated quality parameter and the estimated number of channel filter taps to determine a number of pre-filter taps, if any, to be used in the pre-filter. In addition, the controller evaluates the estimated quality parameter and the estimated number of channel filter taps to determine a number of equalizer taps to be used in the equalizer where the number of equalizer taps is less than or equal to the estimated number of channel filter taps.